Toner concentration monitoring apparatus utilizing programmable digital computer

ABSTRACT

Apparatus for continuously monitoring the concentration of toner in an electrographic developer mixture of toner and carrier particles at a magnetic brush development station by sensing the reflectivity of such mixture. The apparatus includes a radiation source for illuminating the mixture, first and second photocells for producing analog signals representative of the reflectivity of the mixture and the intensity of the radiation source, respectively. The apparatus further includes digital processing apparatus having an analog to digital converter and a programmable digital computer, having stored program, which in response to the analog signals, produces in accordance with such stored program a representation of the relative proportion of toner particles in the mixture.

United States Patent Powell et al.

I TONER CONCENTRATION MONITORING APPARATUS UTILIZING PROGRAMMABLEDIGITAL COMPUTER [75] Inventors: Stephen R. Powell; John L. Connin,

both of Rochester. NY.

[73) Assignee: Eastman Kodak Company,

Rochester. NY.

[22] Filed: Oct. 1,1973

[2i] Appl No.: 402,222

[52] US. 222/57; 222/DIG. 1; ll8/637 [51] Int. Cl B67d 5/56 [58] Fieldof Search 222/57, 76, DIG. 1;

{56] References Cited UNlTED STATES PATENTS 3.742.489 6/[973 Lcfevre etal 340/347 AD 3.777.173 [2/1973 Lundrith POSITIVE BIASED A PWR. SPLY.

NEGATIVE PWR. SPLY.

Apr. 8, 1975 [57] ABSTRACT Apparatus for continuously monitoring theconcentration of toner in an electrographic developer mixture of tonerand carrier particles at a magnetic brush development station by sensingthe reflectivity of such mixture. The apparatus includes a radiationsource for illuminating the mixture. first and second photocells forproducing analog signals representative of the reflectivity of themixture and the intensity of the radiation source. respectivelyv Theapparatus further includes digital processing apparatus having an analogto digital converter and a programmable digital computer. having storedprogram. which in response to the analog signals, produces in accordancewith such stored pro gram a representation of the relative proportion oftoner particles in the mixture.

5 Claims, 4 Drawing Figures COPY HANDELING ACCESSORY 3| :2 l 34 I} camel we STATIO DIGITAL COMPUTER HEP-ITEM @7335 "875,106

SEIEET 1 BF 3 co 36 DE 6 PO VE 26 ESSORY BIA AC.

PWR. SPLY. 32 34 NEGATIVE 25 l 29 @Q ia PWR. SPLY. I

& @wo w CLEANING STATION CHARGING STATION COMPUTER 74 FIG.|

' CLOCK I MULTIPLEXER V I I 96 74 REPL.

COMPUTER CONV V CIRCUIT SIGNAL M OSCILLATOR PROCESSING CIRCUIT I ISIGNAL FIG. 2

PROCESSING CIRCUIT TONER CONCENTRATION MONITORING APPARATUS UTILIZINGPROGRAMMABLE DIGITAL COMPUTER CROSS REFERENCE TO RELATED APPLICATIONSReference is hereby made to commonly-assigned United States patentapplication Ser. No. 280,397 filed Aug. I4, 1972, entitled, TONERCONCENTRATION AND AUTO BIAS CONTROL APPARATUS, in the name of ConradAltmann; and commonly-assigned U.S. Pat. No. 3,830,401, issued Aug. 20,1974, entitled, TONER CONCENTRATION MONITORING APPA- RATUS, in the nameof Bruce Benwood et al.

BACKGROUND OF THE INVENTION 1. Field of the Invention The inventionrelates to apparatus for controlling the concentration of electroscopictoner particles in an electrographic development mixture. Morespecifieally, this invention relates to toner monitoring apparatus ofthe type wherein toner concentration is determined by sensing thereflectivity of the developer mixture.

2. Description of the Prior Art In the electrographic reproductionprocess, the surface of a radiation-sensitive photoconductive memberwhich may comprise a layer of photoconductive material disposed on aconductive backing, is given a uniform electrostatic charge and is thenimage-wise exposed to a pattern of aetinic radiation corresponding tothe indicia on a document or the like being reproduced. Such exposureserves to selectively dissipate the uniform charge on the surface,leaving behind a latent electrostatic image which can then be developedby contacting it with an electrographic developer.

In general, eleetrographic developers comprise a mixture of suitablypigmented or dyed resin-based electroscopic particles, known as toner,and a granular carrier material which functions to carry such toner bygenerating triboelectric charges thereon. The development of the latentelectrostatic image occurs when the developer mixture is brought intocontact with the electrostatic image-bearing surface. Such contact iscommonly effected by either cascading the mixture over such surface orsubjecting the surface to the periphery of one or more rotating magneticdevelopment brushes, the bristles of which comprise chain-like arrays oftoner-coated carrier particles. Upon contacting the electrostaticimage-bearing surface, the toner particles, being charged to a polarityopposite to that of the electrostatic image, are separated from thecarrier particles and are selectively deposited in an imagewiseconfiguration on the surface to form a developed or toner image whichmay thereafter be transferred to a paper receiving sheet and fixedthereto by any suitable means, such as heat, to form a copy of theoriginal document. As toner images are repetitively formed, tonerparticles are continuously depleted from the development mixture,requiring subsequent replenishment to avoid a gradual reduction in imagedensity.

To avoid the necessity of manual replenishment and the operatingdifficulties often encountered as a result of over-replenishment, avariety of devices has been heretofore proposed for automaticallyreplenishing toner particles after a predetermined number of copies aremade or, alternatively, after the concentration of toner particles inthe developer mixture drops below a predetermined level. Exemplary ofphotoelectric or optical toner concentration monitoring devices is thedevice disclosed in U.S. Pat. No. 3,233,781 issued to W. J. Grubbs whichutilizes the difference in reflectivity exhibited by toner and carrierparticles as a means for monitoring the concentration of toner particlesin the developer mixture. Toner particles, usually being black andpossessing highly absorbing surfaces, reflect less radiant energy thanthe carrier particles. Thus, the reflectivity of the developer mixturedepends upon the relative proportions of the mixed particles. Accordingto the Grubbs disclosure, the reflectance of the developer mixture ismonitored by directing energy emanating from an incandescent lamp towardthe mixture and detecting the energy reflected by the mixture with aphotoconductive cell. Such photocell, together with a similar photocellwhich is illuminated directly by the lamp and thereby acts as areference signal, is employed as a variable resistance arm of a bridgecircuit which is capable of activating a toner replenishing device inresponse to a predetermined change in the ratio of photocell outputs,such change being characterized by an unbalance in the circuit.

While operable, photoelectric toner monitoring devices of the typedescribed above have not proven entirely satisfactory in operation,especially over extended periods of time. A principal cause ofunsatisfactory performance is due, at least in part. to the incandescentlamp used for illuminating the development mixture, a luminous energysource of relatively low output, unstable intensity and short life, andthe fact that spectral characteristics of the lamp are closely akin tothe background light, thereby making background discriminationdifficult. The reference photocell and bridge circuit of the Grubbsapparatus provide a means to compensate for fluctuations in theintensity of the output of the lamp itself. Nevertheless, a source ofuncompensated error in the Grubbs apparatus is the random electricalperturbations, commonly called noise", which are received and generatedby both the detecting and reference photocells. It is difficult inanalog control systems to conveniently provide a high degree of noiserejection. Another problem associated with existing optical tonerconcentration monitors is that if a manufacturer should desire to changethe constituents of either the photoconductor or the developer mixture,then most probably this would require some significant redesign of theanalog circuitry.

SUMMARY OF THE INVENTION It is therefore an object of the presentinvention to improve photoelectric devices adapted for monitoring andmaintaining substantially constant the relative proportion of mixedmaterials having divergent reflectance characteristics.

Another object of the invention is to provide an apparatus whereinsystem parameters can be readily altered to accommodate developmentchanges without incurring hardware redesign alteration costs.

A still further object of the invention is to provide a tonerconcentration apparatus which makes use of the advantages of digitalcomputation.

A still further object of the invention is to provide tonerconcentration apparatus which facilitates noise rejection.

These and other objects of the invention are achieved by the provisionof a substantially improved apparatus for monitoring. during theoperation of an electrographic copier, the reflectivity of the developerused therein.

In the disclosed apparatus. there is provided a source of illuminationfor illuminating a developer mixture of toner and carrier particles. Theapparatus includes first and second photosensitive means the first beingdisposed to receive radiation from said source reflected from themixture and the second being disposed to receive radiation directly fromsaid radiation source. The first and second photosensitive means producefirst and second analog signals representative of the reflectivity ofthe mixture and the intensity of the radiation of said sourcerespectively.

The apparatus further includes a digital processing apparatus whichincludes a programmable digital computer having a stored program whichin response to such first and second analog signals produces inaccordance with such stored program a representation of the proportionof toner particles in the mixture. By utilizing a programmable digitalcomputer. if there should be changes in the developer mixture forexample. then these changes can be accommodated for by changes only inthe software program for the computer thereby eliminated the need byhardward redesign.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic vertical sectionof an electrophotographic apparatus wherein the invention may beembodied;

FIG. 2 is a block diagram of the toner concentration monitor 70 depictedin FIG. 1:

FIG. 3 is a partial perspective view showing in detail a portion of thephotoconductive web and bimorph sensors also shown in FIG. I; and

FIG. 4 is a detailed electrical schematic of electrical circuitrydepicted in block form in FIG. 2.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT To assist theunderstanding of the present invention, the operation of anelectrographic copying machine in which the invention may be used willbe briefly described. It is to be understood, however. that theapparatus of the present invention could be used with equal facility andadvantage in other copying machines, and. therefore. that the followingdescription of apparatus related to but not forming part of theinvention is provided for illustrative purposes only.

Electrographic Copy Apparatus Reference is now made to FIG. 1 whereinvarious stations of an electrographic apparatus are schematicallyillustrated. As in some clectrographic copiers, an information medium 13such as a document is illuminated by radiation from flash lamps 14. Suchradiation is reflected from the medium and projected by a lens 15 onto aphotosensitive web member 16 to selectively dissipate charge and form anelectrostatic latent image. The photosensitive web member 16 which isrelatively transparent may include a photoconductive layer with aconductive backing on a polyester support. The photoconductive layer maybe formed from. for instance. a heterogeneous photoconductivecomposition such as disclosed in the commonly assigned US. Pat. No.3.6l5,4l4, issued Oct. 24, 197]. The web member 16 is trained aboutdrive rollers 4 through 9 and is uniformly charged at a charging station20 with a negative DC charge. Assuming that the information on thedocument 13 is black on a white background. the photoconductive layer ofweb members 16 is rendered conductive in areas corresponding to theoriginal background. leaving a latent image of negative charge only inareas corresponding to the original black image.

The apparatus 10 further includes a magnetic brush development station22 at which the moving electro static image is contacted by tonerparticles formed from a fine thermoplastic powder which adheres byelectrostatic attraction to the negatively charged portions of theelectrostatic image to develop and render such image visible in animage-wise configuration. A post-development erase lamp 23 thenilluminates the photoconductive layer of web 16, through the relativelytransparent support and conducting layers to reduce photoconducterfatigue (deterioration resulting from prolonged charge) and tofacilitate subsequent toner image transfer. The toner remains in itsimagewise configuration on the surface of the web member 16 by adhesiveand other forces of attraction.

A transfer station 24 is provided to cause toner particles to betransferred in an imagewise configuration to a receiving surface ofacopy sheet of paper which is fed from a selected one of two papersupplies 21 and 27 through a registration device 19 and then onto thesur face of the web member 16 at the transfer station 24. The transferstation includes a negatively charged DC corona device 25 that applies anegative charge to the back of the paper which draws the paper byelectrostatic attraction into intimate contact with the web member 16.Due to the charge gradient between the paper and the photoconductivelayer of web member 16. the toner on the web member is transferred in animagewise configuration onto the paper. The paper and web member 16 thenmove under an AC corona device 26 which removes the charge from thepaper and renders it virtually neutral in charge. A positive bias isapplied to the AC power supply for the corona 26 to overcome thetendency of balanced AC corona devices to produce a negative charge.

When the paper reaches the position on the web member 16 just above theroller 8, the web member bends sharply around the roller and the beamstrength of the paper coupled with the momentum of the moving papercauses the paper to continue in an essentially straight-line path andleave the web member 16. A vacuum transport member 29 is located abovethe photoconductor at this point to convey the paper to move in suchstraight-line path into a fusing station 30. At the fusing station 30,the toner is heated and fused to the paper to provide a finalsubstantially permanent copy. This copy follows either of the pathslabeled 31 or 32 in exiting from the machine to either a hopper 34 orcopy handling accessory 36 such as a sorter. Finally, a cleaning station17 is provided in which residual toner is removed from thephotoconductive layer of the web member 16 prior to charging. In certainknown modifications of this same apparatus, one or more of thesestations may be eliminated or modified. For a more complete descriptionof the general organization of such as electrographic apparatus,reference is made to commonly-assigned U.S. Pat. No. 3,746,443 issuedJuly 17, 1972. entitled, MAGNETICALLY CON- TROLLED MACHINE PROGRAMMER,to Hickey.

Development Station The development station 22 of theelectrophotographic apparatus described hcreinabove is of the magneticbrush variety, commonly comprising a trough 50 for containing anelectrographic developer 51 and a pair of conventional magneticdevelopment brushes 52 and S3 for applying the developer to theelectrostatic image-bearing surface of web member 16. Each brushgenerally includes a rotatable mounted aluminum cylinder 54 having atleast one magnetic polc piece 55, interiorly disposed in a fixedposition along the longitudinal axis thereof. As previously mentioned,electrographic developer 51 generally comprises a mixture of toner andcarrier particles which adhere to each other under the influence oftriboelectric forces. In magnetic brush development. the carrierparticles are fabricated from a magnetically attractable material so asto be attracted to the surface of cylinder 54 by the magnetic fieldproduced by the internal pole pieces 55 to form chain-like arrays whichmay resemble the bristles of a brush. A pair of rotating mixing augers58 and 59 serve to continuously circulate the developer laterallythrough trough 50 and maintain the relative concentrations of thedeveloper components substantially constant throughout the trough.Typically, drive means (not shown) are provided for rotating thedevelopment brushes in a direction such that the movement of the brushesin the vicinity of contact with web member 16 is in a direction oppositethe direction of travel of such web.

As cylinders 54 rotate, developer 51 collects on the outer surfacesthereof under the influence of the magnetic field produced by theinternal pole pieces 55. As the electrostatic image-bearing surface ofweb member 16 tangentially contacts with the developer-bearingcylinders, toner is stripped from the carrier particles. due to thestronger electrostatic forces, and selectively deposited on the webmember to form toner images. As the cylinders 54 continue to rotate, thepartially denuded carrier particles used in forming the toner images aremoved beyond the influence of pole pieces 55 and fall back into the mainbody of the developer to be recoated with toner. As successive tonerimages are formed, the concentration of toner in the developer graduallydiminishes. the ultimate result being toner images of graduallydecreasing density. An example of an exemplary magnetic developmentstation 22 is more fully disclosed in commonly assigned U.S. Pat. No.3,543,720 to Drcxler et al.

A conventional toner replenisher 60 is activated by a tonerconcentration monitor (to be described) to maintain the concentration oftoner at or above the level required for high quality copies. Thereplenisher 60 includes a hopper 61 which defines an opening in which isdisposed a paddle wheel 62 which may also be in the form of a brush orsoft fibrous roller. The wheel 62 is driven by a motor 66. When themotor 66 is energized, it rotates the paddle wheel 62 causing toner tobe fed into the trough 50 thereby increasing the concentration of tonerin the developer. An example of a conventional toner replenisher is setforth in U.S. Pat. No. 3,409,901. Since the carrier and toner componentsof the developer commonly possess divergent reflectance characteristics,with toner having a lower reflectivity than the carrier component, thereflectance of the developer 51 is an accurate measure of the relativeconcentrations of its two major components. The

higher the concentration of the carrier particles, the higher thereflectance of the developer. Stated otherwise, the developer 51reflectance is inversely proportional to the toner concentration.

Toner Concentration Monitor Apparatus In accordance with the invention,there is provided improved toner concentration monitor apparatus 70 tocontinuously monitor the concentration of toner in the mixture 51.Monitoring of the reflectance of the developer 51 is accomplished bydirecting the output of radiant energy source 71, preferably a lightemitting diode, toward a portion of the developer where theconcentration is representative of the average toner concentrationthroughout the developer. In the magnetic brush development station 22,the toner concentration on the downstream development brush (i.e., brush52) between the points where the brush surface emerges from thedevelopment mix and first contacts the surface of web member 16 isusually characteristic of the average toner concentration. Between suchpoints, the surface of the development brush comprises fresh developerwhich has not yet been subjected to the localized depletion of tonerwhich results from development of the electrostatic image.Alternatively, toner concentration can be monitored on the upstreamdevelopment brush (i.e., brush 53) between the points where the brushsurface loses contact with the surface of web member 16 and re-entersthe developer. To eliminate the effect of localized toner depletion fromthe surface of the upstream development brush, a comb-like member (notshown) having teeth extending into the brush surface (such a member isdisclosed in the above-cited Benwood et al. Patent Application) can beused to agitate the developer carried by the brush before beingsubjected to the toner monitoring operation.

The use of a light-emitting (LED) diode 71 as the source of radiationfrom which developer reflectance is determined offers the advantagesover conventional sources of long-term stability, collimated output.fast response time. compactness and physical durability. Because ofitsfast response time, the LED can be electronically pulsed at electronicspeeds. The LED 71 is actually periodically energized at a selectedfrequency by means of a conventional pulse generator. Light reflectedfrom off the brush 52 is received and processed by signal processingcircuitry 72. Light from LED 71 is also received directly by signalprocessing circuit 69. Circuits 69 and 72 are shown at this point forthe sake of convenience in the disclosure as applying signals directlyinto a programmable digital computer 74. The circuits 69 and 72 will beunderstood to include filters having band passes centered on thefrequency of the oscillator 73. This will aid in noise rejection. Aswill be seen when FIG. 2 is discussed. these signals are actuallymultiplexed and converted to a digital format prior to being received bythe computer 74. The computer 74 also receives input signals from a pairof electromechanical transducers 76a and 76b (referred to herein asbimorph sensors" which sense perforations in the web member 16. Inresponse to these signals, the computer 74 solves a toner concentrationcontrol equation (later described) and in accordance therewith actuatesthe motor 66.

Reference should now be made to FIG. 3 which shows in detail a portionof the web member 16 having along its border two rows of indicia orperforations 16a and 16b. Between adjacent perforations 16a is definedan image area. By that it is meant an image area is a place across theentire width of the web member 16 wherein a charge pattern correspondingto an image may be placed. The row 16b defines a predetermined number ofequally spaced perforations or sprocket holes disposed between adjacentperforations 160 which too are equally spaced along the web member l6.The distance between adjacent perforations 16a being much greater thanthat between adjacent perforations l6b. As shown, there is provided abimorph sensor 760 which is adapted to sense the perforations 16a andprovide a signal to the digital computer 74, each time a perforation 16ais sensed. A second bimorph sensor 761) is adapted to provide a clockpulse to the computer 74, each time a perforation 16b is sensed. Onlythis clock pulse line is shown in FIGS. 2 and 4 since the pulse from thesensor 74a is not directly concerned with this invention. The reasonboth lines have been shown in FIGS. 1 and 3 is because the computer 74uses the pulses from both bimorph sensors 76a and 76b to control andsynchronize the various work stations of the electrophotographicoperation. In operation. upon receiving a clock pulse from a bimorphsensor 760. the computer 74 enables the start line 96 (briefly seen inFIG. 2) which causes a digital word having six bits to be stored in astorage location which could be by semi-conductor memory devices, notshown. Two words for each clock pulse (corresponding to an increment ofa charged image area) are sequentially accepted by the computer and thenprocessed by the signals produced by circuits 69 and 72 respectively.The computer is programmed to operate upon clock signals from thebimorph sensor 76b and determine the concentration of toner in themixture. This will be described more fully in the sections of thisdisclosure entitled, COMPUTER OPERATION. The computer is also used tocontrol other functions such as synchronizing the operation of the sheetfeeding apparatus and operating the exposure lamps M at appropriatetimes in the machine cycle. The computer 74 may take various other formsknown in the art some of which are commercially available asprogrammable minicomputers and programmable microprocessors. Specificexamples are, Model 8008 Micro-Computer manufactured by IntelCorporation of Santa Clara, California; GEPAC 30 manufactured by theGeneral Electric Corporation; lnterdata Model I or Varian Data MachinesModel 520/1'. The instructions and method of programming suchminicomputers is set forth in the textbook, "Minicomputers for Engineersand Scientists", Gravino Korn (I973 An example of bimorph sensors 76aand 76b which may be suitable for use with the present invention isdescribed in commonly assigned US. Pat. No. 3,723,650 in the name ofBradley et al, issued Mar. 27, 1973, entitled, METHOD AND APPARATUS FORDERIVING THE VELOCITY AND RELATIVE POSITION OF CONTINUOUSLY MOVINGINFORMATION BEARING MEDIA. Briefly, such bimorph sensors include apiezoelectric crystal 82 which has attached thereto a single step sensor83 element, the distal ends 84 of which bear on and slide against themoving web member 16. When a perforation in the member moves beneath thedistal end of the sensor element, the end abruptly drops over theleading edge of the perforation and distorts or otherwise inducesmechanical movement of its associated transducer. As the web member l6continues to move, the distal end 84 of the sensor element 83 is forcedout of the perforation by engagement with the trailing edge of theperforation, and once again the sensor element distorts its associatedpiezoelectric transducer. By means of electrodes or other suitablecurrent collecting means attached to the sensors, voltage signalsgenerated by the distortion of the transducer are transmitted to thecomputer 74. Other types of perforation sensors which produce outputsignals such as optical perforation sensors or other types of ceramictransducers responsive to compression,

bending or other forms of physical distortion may be substituted for thedepicted bimorph sensors.

Turning now to FIG. 2, there is shown a more detailed block diagram ofthe toner concentration monitor in accordance with the invention. Asshown, the oscillator 73 provides a drive current for the LED 7] whichilluminates the brush 52 with light in the infrared portion of thespectrum and having a frequency determined by the oscillator 73. Thespectral region where developer reflectance is monitored should bechosen in accordance with the availability and course require ments ofcommercial light-emitting diodes and photodetectors kept in mind;however, an important requirement is that the wave length at which thereis a reflectance disparity between toner and carrier should be maximizedto the greatest extent possible to provide improved system sensitivity.Light from the brush 52 is reflected to illuminate the signal processingcircuit 72 which includes as a detector device a phototransistoroperating between the base and collecting junctions as a photovoltaiccell. The detected signal is amplified and converted to a DC level bythe circuit 72 and then applied to an analog multiplexer which at theappropriate time, selected by the computer 74, provides such signal toan analog to digital (A/D) converter 92.

Since a variation in the intensity of the light from LED 71 could causea signal to be produced by the circuit 72, which could indicate a wrongtoner concentration, the toner concentration monitor compensates forvariations in light intensity. Towards this end, the circuit 69 receiveslight directly from the LED. In a preferred embodiment, the energyoutput of the LED 7l may be coupled to a detector of the circuit 69 bymeans of a conventional light pipe. By means of this arrangement,problems of toner dust in the optical path may be minimized. The circuit69 detects the light, amplifies and converts it to a DC level in muchthe same manner as the circuit 72. The signal is also applied to themultiplexer 90 which, at the appropriate time feeds its multiplexedoutput to the A/D converter 92 which in turn applies a corresponding sixbit digital word into the computer 74. The arrangement is such that ifthe concentration of toner in the mixture is at the desired level, thenboth analog input signals (from circuits 69 and 72, respectively) willbe at the same amplitude.

In order to process signals from the circuits 69 and 72 the computer 74produces multiplexer channel address select signals to sequentiallyswitch inputs to the AID converter 92. These signals are sequentiallyproduced after a perforation 16b has been detected by sensor 76b.Thereafter, start signals are produced in the lead 96 which causes theA/D converter 92 to perform a conversion of input analog data into sixbit digital words. Upon completion of each such A/D conversion process,the converter produces a completion signal to the computer 74 in a lead98 which causes the computer in the appropriate sequence in its programto accept the data from A/D converter 92. As just noted, the sequencingof the computer is in turn coordinated as a function of the input clocksignals provided by the transducer 82 upon sensing the perforations inthe film. The computer 74 operates upon these input digital words anddetermines the toner concentration. 1f the computer determines toner isto be added to the mixture, it provides an output signal to thereplenishment circuit 75 which in turn energizes the replenishment motor66. The circuit 75 will continue to receive these output signals untilthe computer 74 determines that the toner concentration is back within adesired range. A feature of the invention is that the computer may beprogrammed to monitor the magnitude of toner concentration and provideoutput alert signals if the toner concentration becomes either too highor too low.

In P16. 4, there is shown an electrical schematic diagram of many of thecircuit elements shown in block form in PK]. 2. The oscillator circuit73 will first be described. The oscillator 73 includes an operationalamplifier 100 connected in an astable multivibrating configuration andis adapted to provide an output signal through a line 102 to a resistor103 to the base elec trode of a drive transistor 106. A diode 104 isprovided to protect the transistor 106 from reverse polarity of therecuring signal. The oscillator may for a specific example operate at afrequency of one KHz at a 20 percent nominal duty cycle. The transistor106 operates as an amplifier to provide the required current levels fordriving the LED 71. Capacitor 108 provides energy storage for LED 71 tominimize the power supply loading and reduce noise. The operationalamplifier 100 includes a series of feedback resistors 120, 122, 124 and126, a diode 128, and an oscillator timing capacitor 130. These circuitelements operate in a well-known manner and need not be describedfurther here. For a complete description of wave generators which employoperational amplifiers and for further description of other operationaloscillator circuits which may be used in accordance with the invention,reference is made to Chapter of the Burr-Brown Operational Amplifiers"book (1971) by Tobey et 211., published by McGraw-Hill, New York (1971).

The detecting and signal processing circuit 72 which receives lightreflected from the brush 53 will now be described. A detector for suchcircuit 72 is shown as a phototransistor 134 which may be, for example,a Fairchild FPT-IOO Phototransistor. The phototransistor producescurrent in response to the reflected infrared energy from the developermixture to drive the inverting input of an operational amplifier 140.The phototransistor 134 operates as a photovoltaic cell via itscollector-base junction. As shown, there are three cascaded operationalamplifiers 140, 142 and 146, in circuitry 72 which raise the signallevel of the one KHz signal to say, for example, a minus 6 volt peak.The operational amplifier 146 also acts as a demodulator in combinationwith the diode 150 and capacitor 152 to convert the signal to a nominalDC level of, about 4 volts. To effectively utilize the analog to digitalconverter 92, only that portion of the concentration signal thatrepresents the excursion range is to be digitized. For a specificexample, for a minus 4 volt DC 1: 0.1 volts DC which represents thelimits of toner concentration control, only the AC excursion or 0.2 voltneed be digitized to cover the full range of 0-5 volts of the converter.To accomplish this function, a further operational amplifier 150 isprovided at the output of the multiplexer 90. This technique wouldordinarily raise the nominal level of the signal beyond the range of theconverter 92. Therefore, the signal is offset by the subtracting actionof a 15 volt source coupled to resistors 154 and 156 which are disposedat the input to the multiplexer 90. In this manner, the amplifiedcontrol signal applied to the A/D converter 92 can be shifted back tothe centerpoint (2.5 volts) of the 0-5 voltage range of the converter.It should be noted that at the other input to the multiplexer there isprovided an identical offset voltage producing circuit having resistors154 and 156.

Turning back again to the input portion of the circuit 72, theoperational amplifier serves as a current-tovoltage transducer forconverting the current from the phototransistor 134 to a voltage signal.A resistor 160 sets the gain of the amplifier 140 and also a feedbackcapacitor 164 is provided. A resistor 162 and a capacitor 163 coupledthe first and second operational amplifier. The second operationalamplifier 142 also includes feedback resistor 166 and is coupled by acapacitor 168 to the inverting port of the operational amplifier 146 viaan adjustable resistor 170. Also, the amplifier 146 includes aconventional feedback resistor 172 and feedback diode 174.

Circuit 69 includes a phototransistor which, like phototransistor 134,may for a specific example be a Fairchild FPT-lOO phototransistor. Lightfrom the LED 71 directly illuminates the phototransistor 180 which iscoupled to the non-inverting port of an operational amplifier 182. Sincethe signal received by the photocell 180 is much higher than the signalreceived by the photocell 134, only a single amplifier stage whichincludes the operational amplifier 182 is needed. At the output of theamplifier 182 is a diode and a capacitor 192 which provides thedemodulation function similar to that described at the output ofoperational amplifier 146. Adjustment of the input signal to themultiplexer 90 is accomplished by means of a potentiometer 194 whichchanges the gain of an amplifier 150. The level of this signal isadjusted to be substantially equal to the nominal concentration controlvoltage (2.5 volts DC) as also described in connection with circuit 72.In operation, amplifier 182 converts current pulses to voltage pulses ofa proportional amplitude. The overall gain of the amplifier isdetermined by feedback resistors 200, 202 and 204 and potentiometer 205.The amplifier 182 also includes conventional feedback diode 210 andcapacitor 212.

It should be noted that the multiplexer 90 has been shown only in blockform since it is of a type which may be readily purchased commercially.An example of a multiplexer which has been found to performsatisfactorily is the lntersil Model 1H 5010. Similarly, numerous analogto digital converters are available and one which is especially suitablefor use with the invention is Model ADC-ECONOVERTER manufactured byDate] System, Inc., of Canton, Massachusetts.

The replenishment circuit 75 will now be described. In accordance withthe invention, to operate the replenishment motor 66 an output line 230from the computer 74 is grounded or de-energized. Energizing the line230 is effective to turn the motor 66 off. Towards this end, the circuit75 includes two PNP transistors 232 and 234. The lead 230 is directlycoupled through a resistor 236 to the base electrode of the transistor232. Transistor 232 turns to an energized condition when the voltageapplied on lead line 230 is grounded by forward bias in the base-emitterjunction of the transistor 232. At such time, due to the forward voltagedrop across diode 234, which reverse biases the base-emitter junction oftransistor 234, the transistor 234 is held in a turned-off condition.The transistor 234 is adapted to provide a braking function for themotor 66. With the transistor 234 turned off, a voltage is appliedacross the motor 66 sufficient to energize same. When the line 230 isenergized, the transistor 232 turns off. The motor 66 now acts as agenerator having the same polarity as it had while running as a motor.The transistor 234 is forward biased through resistor 242. Therefore, itturns on and short circuits the armature of the motor 66 to ground,thereby stopping the motor. It should be noted that the diode 240 actsas a transient suppressor for any momentary reverse polarity spikescaused by armature brush switching. The amount per unit time of tonerdispensed by using this technique is a constant when the motor 66 isenergized. This factor is recognized by equation 3 in the controlparameters of the computer described hereinafter. Computer Operation Itis a feature of this invention that the calculations and decisionprocesses concerning the concentration of toner in the developer aredone in the programmable computer 74. Thus, changes are made in thedeveloper, they can be accommodated by making suitable alterations tothe computer software without changing any of the hardware of themonitor 70. The computer 74 should be programmed to solve equations (1(2), and (3) hereinafter set forth. Such a program is clearly within theskill of the programmer having ordinary skill in the art and hence thedetails of such program need not be given here. lt should be noted,however, that the computation process is initiated each time a clockpulse from the sensor 76b is received by the computer 74.

1. Control Parameters: The following basic equations describe thecontrol function:

(2) E aR aC wherein:

V, output voltage to control replenisher Kym controller proportionalgain constant K, controller integral gain constant E concentration errorat)? system reference concentration voltage or set point (If systemdetected concentration voltage at any given time KP)"; 045 controlvolts/unit error K, 0.10 control volts/sec./unit error As noted above,the present invention utilizes a pulse duration modulation technique ofcontrol. The amount of toner being dispensed is therefore constant whilethe replenisher mechanism is energized and the toner dispensing rate iscontrolled by the time period the replenisher mechanism is turned ON.The V voltage of equation l) is converted to time or duty cycle asfollows:

0" r 0 T wherein:

i time replenisher is ON per cycle K replenisher control gain T timeproportioning base period K 0.2 duty cycle/control volts T= l secondReplenisher OFF if t Replenisher ON duty cycle if t T 2 ComputerProcessing of Data: The computer 74 receives two signals as data inputsrespectively produced by circuits 72 and 69. The first signal representsthe reflectivity of the developer at any given time and the secondsignal is a reference representing the energy output level of the LED 71at any given time. To enable the computer 74 to compensate forvariations in LED energy output, the concentration signal is firstdivided by the reference signal to obtain a normalized value of unitythat represents a nominal known system condition. If the referencesignal shifts due to aging of the LED or ambient temperature changes,the concentration voltage shifts by the same amount and the normalizedcontrol value remains unaffected. Therefore, any changes in thenormalized signal represent only toner concentration level changes.

The normalized signal is next subtracted from the desired set point ornominal control point value to create an error signal E. This errorsignal is used in the basic system control equation (1) for theproportional integral controller.

The computer numerically integrates the error voltage in a well knownmanner by simple addition and accumulation of the error signals for eachdata sampling period. The sampling rate for this system is once persecond. The integrated value is then multiplied by the K constant ofequation (1). This value is summed with the error signal multiplied bythe K constant of equation (I) to obtain a V value.

The computer is adapted to determine the time duration for actuation ofthe replenisher mechanism by use of equation (3). This computation isconverted to the number of clock pulse periods that the replenisherdrive mechanism is held energized.

As an auxiliary function the computer may continually compare the valueof the digitized concentration voltage with two stored constant valuesin computer memory. These two constant values represent upper and lowerlimits of concentration allowed by the system. lf the concentrationvoltage fails to remain within these limits the computer provides anoutput alarm signal for activating a warning device (not shown) forappropriate action by the system.

The toner concentration monitor disclosed herein has been found to beextremely sensitive to minute changes in the reflectivity of the mixturebeing monitored, and exceptionally stable over extended periods ofoperation. For these reasons, it has been found useful in maintainingtherelative proportions of mixed materials substantially constant overextended periods even when the reflectance disparity of such materialsis very small. For instance, a change in toner concentration of from 0to 7 percent (7 percent toner concentration producing total saturationor coverage of the carrier particles with toner) results in a 25 percentchange in the reflectivity of the developer mixture. Since, for manydevelopers, it is desirable to maintain the toner concentration withinthe range of 2.75 percent to 3.75 percent, it may be appreciated thattoner concentration monitors must be sufficiently sensitive to detectchanges in the reflectivity of the developer of the order of l percent.By taking into account the changes in the density of the atmosphericenvironment in which the monitoring apparatus is employed as well as thegradual accumulation of mixed materials on the external surfaces of suchapparatus, the reflecting monitoring systems have also been found topossess the stability required for maintaining the toner concentrationwithin the required range for several months of continued use.

The invention has been described in detail with reference to a preferredembodiment thereof, but it will be understood that variations andmodifications can be effected within the spirit and scope of theinvention.

We claim:

1. In apparatus for monitoring in a developer the proportion of carrierand toner particles having divergent reflectance characteristics, theimprovement comprising:

a. a radiation source for projecting radiant energy upon the mixture toilluminate a portion thereof;

b. first and second photosensitive means, said first photosensitivemeans being disposed to receive radiation from said source which isreflected from the mixture and said second photosensitive means beingdisposed to receive radiation directly from said source, said first andsecond photosensitive means being adapted to produce first and secondanalog signals representative of the reflectivity of said mixture andthe intensity of radiation of said source, respectively;

c. analog to digital conversion means responsive to said first andsecond analog signals to respectively produce first and second digitalsignals corresponding thereto: and

d. programmable digital computation means having a stored program andbeing responsive to said first and second digital signals for producingin accordance with said stored program a representation of theproportion of toner particles in the mixture.

2. The invention as set forth in claim 1 including actuable tonerreplenishing means for adding toner particles to the developer andwherein said programmable digital computation means actuates saidactuable toner replenishing means when the proportion of toner particlesin the developer is below a predetermined level.

3. The invention as set forth in claim 2 including a multiplexerresponsive to said first and second analog signals for multiplexing suchsignals to said analog to digital converter means.

4. In apparatus for monitoring the proportion of a mixture of carrierand toner particles having divergent reflectance characteristics, theimprovement compris ing:

a. a radiation source for projecting radiant energy on the mixture toilluminate a portion thereof;

b. first and second photosensitive means, said first photosensitivemeans being disposed to receive radiation from said source which isreflected from the mixture and said second photosensitive means beingdisposed to receive radiation directly from said source, said first andsecond photosensitive means being adapted to produce first and secondanalog signals representative of the reflectivity of said mixture andthe intensity of radiation of said source, respectively;

c. a multiplexer responsive to said first and second analog signals forsequentially multiplexing such signals;

d. an analog to digital converter responsive to said sequentiallymultiplexed analog signals to produce first and second digital signalscorresponding thereto; and

e. a programmable digital computer having a stored program and beingresponsive to said first and second digital signals for producing inaccordance with said stored program a representation of the proportionof the toner particles in the mixture.

5. The invention as set forth in claim 4 wherein said program is adaptedto solve the following control equations:

:1. V, K E K f E d r b. E= aR-aC, and

0" F Q wherein:

V,, output voltage to control replenisher K controller proportional gainconstant K, controller integral gain constant E concentration error aRsystem reference concentration voltage or set point (16 system detectedconcentration voltage at any given time time replenisher is ON per cycleK replenisher control gain T= time proportioning base period

1. In apparatus for monitoring in a developer the proportion of carrierand toner particles having divergent reflectance characteristics, theimprovement comprising: a. a radiation source for projecting radiantenergy upon the mixture to illuminate a portion thereof; b. first andsecond photosensitive means, said first photosensitive means beingdisposed to receive radiation from said source which is reflected fromthe mixture and said second photosensitive means being disposed toreceive radiation directly from said source, said first and secondphotosensitive means being adapted to produce first and second analogsignals representative of the reflectivity of said mixture and theintensity of radiation of said source, respectively; c. analog todigital conversion means responsive to said first and second analogsignals to respectively produce first and second digital signalscorresponding thereto; and d. programmable digital computation meanshaving a stored program and being responsive to said first and seconddigital signals for producing in accordance with said stored program arepresentation of the proportion of toner particles in the mixture. 2.The invention as set forth in claim 1 including actuable tonerreplenishing means for adding toner particles to the developer andwherein said programmable digital computation means actuates saidactuable toner replenishing means when the proportion of toner particlesin the developer is below a predetermined level.
 3. The invention as setforth in claim 2 including a multiplexer responsive to said first andsecond analog signals for multiplexing such signals to said analog todigital converter means.
 4. In apparatus for monitoring the proportionof a mixture of carrier and toner particles having divergent reflectancecharacteristics, the improvement comprising: a. a radiation source forprojecting radiant energy on the mixture to illuminate a portionthereof; b. first and second photosensitive means, said firstphotosensitive means being disposed to receive radiation from saidsource which is reflected from the mixture and said secondphotosensitive means being disposed to receive radiation directly fromsaid source, said first and second photosensitive means being adapted toproduce first and second analog signals representative of thereflectivity of said mixture and the intensity of radiation of saidsource, respectively; c. a multiplexer responsive to said first andsecond analog signals for sequentially multiplexing such signals; d. ananalog to digital converter responsive to said sequentially multiplexedanalog signals to produce first and second digital signals correspondingthereto; and e. a programmable digital computer having a stored programand being responsive to said first and second digital signals forproducing in accordance with said stored program a representation of theproportion of the toner particles in the mixture.
 5. The invention asset forth in claim 4 wherein said program is adapted to solve thefollowing control equations: a. V.sub.o = K.sub.Pro E + K.sub.INT.intg.E d t b. E = .alpha.R - .alpha.C, and c. t.sub.on = K.sub.cV.sub.o T